This invention relates to a method of measuring the z-directional (out of plane) tensile strength of paper or board, as well as to a device applicable in such a method.
The z-directional tensile strength of a paper or board, i.e. the internal tensile strength thereof in a direction perpendicular to its surface, the thickness direction, may be defined as the maximum tensile stress at break of a defined test area of the material in a plane between its upper and lower surface when loaded in an out of plane mode under defined standard conditions.
There is a multitude of terms referring to the tensile strength in the z-direction of a paper or board. Depending on the method and purpose of measurement as well as the type of sample tested, terms such as z-directional tensile strength, Scott Bond, internal bond strength, internal fiber bond strength, ply adhesion and ply bond strength have been used.
The z-directional tensile strength of a paper or board is for many reasons a critical parameter thereof, which often has to be specified by the manufacturer. By way of example, in processes of applying coatings such as sizings or printing inks to a sheet of paper or board, too low a value of the z-directional tensile strength may lead to the pulling apart of the sheet by adhesion to the applicator, such as a roll, causing build-up thereon of the paper or board.
Also, out of environmental concern there is an increasing demand for recycled paper fibre. However, the higher the content of recycled fibre is in the paper or board, the more difficult it is to obtain an adequate bonding together of the internal layers of the material and thus the more difficult it is to obtain a satisfactory tensile strength. In this case, to ensure a high quality material, frequent controls of the z-directional tensile strength are necessary.
Too high a z-directional tensile strength may also cause problems, since this may lead to inadequacy of other properties of the paper or board, such as folding stiffness or tear strength.
In view of the importance of the z-directional tensile strength as a quality parameter of a paper or board, standards have been developed for its testing, as well as devices applicable in such testing.
In the United States, the standard for testing the z-directional tensile strength of paperboard is given in TAPPI 541, as described in T 541 om-89, incorporated herein by reference and which may be found in TAPPI TEST METHODS 1996-1997, ISBN no. 0-89 852-334-6, ed. TAPPI PRESS, Atlanta, US. The testing procedure consists of applying double (two-sided) coated, pressure-sensitive tape to both sides of a test piece. The test piece then is placed between two platens and compressed uniformly over the entire test area. Finally, uniform tension is applied over the entire test area in a direction perpendicular to the plane of the test piece (z-direction) to effect a separation.
As defined in the TAPPI 541 standard, the testing apparatus comprises a z-direction tensile-compression tester; double (two-sided) coated pressure sensitive tape, 50 mm wide; paper cutters or scissors; and 10 kg calibration weights. The tensile-compression tester is equipped with a force measuring device of 90 kg minimum capacity; 6.45 cm2 tester platen area and a self-adjusting test head (which compensates for any lack of surface parallelism), and provides for a test cycle consisting of a compression stroke, a dwell time (to assure an adequate tape bond between the test piece and the tester platens) and a tension stroke (which effects the splitting of the material, leaving a portion of the material on each of the tester platens). The complete test cycle requires approximately 45 seconds.
A tensile-compression tester conforming to the TAPPI 541 standard is sold by Testing Machines Inc., of the USA, under the name of Monitor/ZD(trademark).
In the Scandinavian countries, the Scandinavian Pulp, Paper and Board Testing Committee recently has put forward a new standard for the testing of the z-directional tensile strength. This standard is to be published in 1998 and will be referred to as SCAN P80:98. In its basic principle, the new standard conforms to the TAPPI 541 standard, however with different values of testing parameters such as the test area and the compression force. Said standard as published is incorporated herein by reference.
Thus, the apparatus according to the proposed Scandinavian standard comprises a compression device with a compression force of (3xc2x10.5 MPa); a tensile apparatus; two flat circular tester platens having a test surface area of (10xc2x10.1 cm2); a device for aligning the tester platens; self adhesive tape, double (twosided) coated; punch or cutter for preparation of test pieces; and solvent.
The test pieces, with a diameter of at least 50 mm or, if square, no side shorter than 50 mm, according to this standard is to be cut from specimens of undamaged paper or board.
The detailed procedure starts with the cleaning of the tester platens with a solvent such as ethyl alcohol or similar prior to each day""s testing. It is of uttermost importance that the test surface of the platens then be maintained perfectly clean.
Next, the protective liner is removed from one side of pieces of the double (two-sided) coated self adhesive tape which are then applied to each side of the test piece. The other protective liner then is removed from the adhesive tape on each side of the test piece, and the whole sandwich structured test piece is placed between the aligned tester platens so that the test piece protrudes outside the tester platens by at least 4 mm around the whole circumference. The whole assembly of tester platens and test piece is placed in the testing apparatus comprising a compression and tensile device, a force sensor as well as self-adjusting heads compensating for any lack of parallelism of the tester platens and the test piece.
After the test area has been compressed with a specified compression force during a specified dwell time between the platens, it is strained to break; the recorded maximum force corresponds to the z-directional tensile strength.
When finally removing the test piece from the tester platens, care should be taken not to touch the test surface of the platens, or else the surface has to be cleaned again with solvent and dried.
A tensile-compression tester conforming to the proposed Scandinavian standard is sold by Lorentzen and Wettre, under the name of Alwetron(trademark).
According to this standard, at least 5 determinations should be performed to get a statistically significant result.
From the above description of the testing method it will be apparent that in order to obtain a reliable value of the z-directional tensile strength, quite cumbersome testing procedures have to be performed involving a substantial amount of manual work comprising the cutting to pieces of the material to be tested and of the adhesive tape, the mounting and taking apart of the test assembly comprising the tested material, the tape and the tester platens. Moreover, it is in fact often not easy to avoid touching the surface of the platens in order to remove every trace of residue after a test, and consequently, frequently the use of a solvent is required in order to clean the tester platen surface from e.g. hand grease, which is a drawback from an environmental point of view as well as a health hazard to the person performing this cleaning, being exposed to the solvent.
Moreover, the manual application of the adhesive tape to the material not always is devoid of difficulties, such as forming of wrinkles in the tape. A test piece bearing an imperfectly applied tape has to be discarded.
Thus, the testing method, due to the large amount of manual work, is quite slow, being both cumbersome and time-consuming, whereas the production of the paper or board in itself is a fast process.
Consequently, there is a need for a method of testing the z-directional tensile strength of paper or board, as defined in the above standards, fulfilling the requirements of being faster and more easily performed by involving less of manual operations as well as providing accurate z-directional tensile strength values in a repeatable way.
It is an object of the present invention to meet said need.
For the purpose of the invention the following terms will have the meaning as defined herein below:
Test area: the surface area whereupon the z-directional tensile strength test is performed, i.e. corresponding to the test surface area of the tester platen. E.g. according to the TAPPI standard the test area is 6.45 cm2 whereas according to the SCAN standard it is 10 cm2.
Test piece: a piece of paper or board originating from a sheet of paper or board, e.g. from a jumbo roll produced in a paper mill, whereupon a number of z-directional tensile strength tests are to be performed.
Sample: a number of nxe2x89xa71 test pieces, which, in case n greater than 1, are assembled in a sequence so as to give a piece of greater length, whereupon a number of z-directional tensile strength tests are to be performed. Obviously, a test piece is equivalent to a sample having n=1.
Sample web: a sandwich structure consisting of a test piece (n=1) or a sample (nxe2x89xa71) having double-sided pressure-sensitive adhesive tape applied to both sides.
The object of the invention is achieved by a method and a device for measuring the z-directional tensile strength as defined in the appendant claims.
The method of measuring the z-directional tensile strength of a sheet of paper or board according to the invention is essentially characterized in that a number of z-directional tensile strength tests may be performed at separate locations on a continuous sample of the sheet, using a sample which permits the carrying out of a number of z-directional tensile strength tests.
According to a preferred feature of the method of the invention, after performing a tensile strength test on a sample web by use of a device of the invention comprising a tensile strength testing apparatus with a pair of tester platens, the tester platens are detached from the sample web while the sample web is retained by the grip of holding means.
According to another preferred feature of the method of the invention, after detaching the tester platens from a test area of the sample web on which a z-directional tensile strength test has been performed, a following section of the sample web is fed into the space between the tester platens.
According to still another preferred feature of the method of the invention, any imperfect parallelism between the tester platens and the sample web when bringing the tester platens in contact with the sample web is compensated for.
According to another preferred feature of the method of the invention, the sample of paper or board to be tested may comprise a sequence of several test pieces attached to each other.
The inventive method is performed by use of the device of the invention, which is essentially characterized in that it comprises means for delivering double-sided pressure-sensitive adhesive tape provided with protective liner on at least one side, means for removing the protective liner from the double-sided pressure-sensitive adhesive tape; means for applying the double-sided pressure-sensitive adhesive tape to each side of a sample of paper or board to be tested, giving a sandwich structured sample web formed of the sample on both sides coated with the double-sided pressure-sensitive adhesive tape; means for feeding the sample web into a z-directional tensile strength tester; a z-directional tensile strength tester comprising a pair of tester platens, of which at least one is mobile in a direction essentially perpendicular to the plane of the test surface the other tester platen to come in contact with the sample web; and means for retaining the sample web, provided on each side of the tensile strength tester.
According to a preferred feature of the device of the invention, the means for feeding the sample web are located on each side of the plane of the sample web, at the exit side of the tensile strength tester.
According to another preferred feature of the device of the invention, the means for removing the protective liner from the pressure-sensitive adhesive tape and the means for applying the pressure-sensitive adhesive tape to the sample, to form a sample web, are located on each side of the plane of feeding of the sample web.
According to still another preferred feature of the device of the invention, means are provided to compensate for any imperfect parallelism of the alignment of the surface of the sample web and the tester platens, respectively.
According to still another preferred feature of the device of the invention, the device is coupled to means for controlling the device and for receiving, storing and treating input as well as output data.
The invention provides an extensively automatized method and device wherein the cutting of the test piece of paper or board is simple to perform and non-critical to the test result; the double-sided pressure-sensitive adhesive tape is automatically applied to both sides of the sample, giving a sample web, which is automatically fed into the tensile strength tester; the tester platens are automatically applied to both sides of the sample web; and the z-directional tensile strength is automatically measured with registration and optionally recording of the exerted force.
The method and device of the invention, by eliminating the cumbersome manual operations of the methods according to the state of the art, as a further beneficial effect will provide a measure of the z-directional tensile strength with a very high degree of accuracy and repeatability.
Moreover, due to the reduced manual handling of the tester platens in the method of the invention, the need of using a solvent to clean the tester platens is substantially reduced, which from an environmental and health protection point of view is very beneficial.